1. Field of the Invention
The present invention generally relates to implantable medical devices and more particularly relates to electrodes for electrically coupling to nerve tissue.
2. Description of the Prior Art
The use of electrodes to monitor electrical activity and stimulate body tissue is quite old. U.S. Pat. No. 1,662,446 issued to R. H. Wappler teaches an early electrode system. The Wappler electrode is used for acute stimulation only, and is not implantable.
An early stimulation electrode which is chronically implantable is taught by S. I. Schwartz, et al in U.S. Pat. No. 3,421,511, herein incorporated by reference. U.S. Pat. No. 3,654,933 issued to Hagfors, herein incorporated by reference, teaches an improved stimulation electrode for chronic implantation. Clinical experience with the electrodes taught by Schwartz, et al and Hagfors may produce excess irritation in certain applications. This irritation results in swelling of the nerve tissue and ultimately an unacceptable increase in impedance of the nerve tissue/electrode interface.
U.S. Pat. No. 4,341,221 issued to Testerman teaches an improved nerve electrode for chronic implantation which uses gel electrodes. While offering some improvement, excess irritation is yet experienced in some patients.
Cardiac pacing leads which sense and stimulate activity in the myocardial muscle tissue have been in use for some time. U.S. Pat. No. 4,711,251 issued to Stokes teaches an endocardial pacing lead having steroid drug embedded in the distal tip. U.S. Pat. Nos. 4,506,680; 4,577,642; and 4,606,118 teach similar endocardial pacing leads, all of which have an embedded steroid drug. U.S. Statutory Invention Registration No. H356 discloses a pacing lead suitable for epicardial application which elutes a steroid drug. Because cardiac pacing leads establish electrical contact with muscle tissue rather than nerve tissue, the total contact area is extremely small to increase current density. Therefore, the area of muscle tissue to be treated by the embedded drug is so small that only minute quantities of drug need be eluted.